1. Field of the Invention
The present invention relates to a display device, in particular, a display device provided with a light-emitting element as a display element. In addition, the present invention also relates to an electronic device provided with the display device in a display portion.
2. Description of the Related Art
In recent years, a technique for forming a thin film transistor (hereinafter referred to as a TFT) over a substrate has drastically progressed and technological development for applying a TFT to an active matrix display device has been advanced. In an active matrix display device, image expression properties with high definition and high gray scale levels have been demanded, and technology for high quality image has been actively developed. In particular, an electroluminescence element (hereinafter, an EL element) that is a light-emitting element shows promise for a display device provided for each of pixels in an active matrix display device in achieving high image quality, because it has a wider view angle than a liquid crystal display device using a liquid crystal element and superiority in color, contrast, and moving-image response speed. Thus, a display device provided with an EL element has been actively developed, and commercialization thereof has been advanced.
However, a transistor for driving an EL element is degraded in proportional to displaying time, and a difference occurs between a gray-scale level attempted to be displayed and a gray-scale level displayed in practice. This is because electrons and holes, which are carries, are trapped in a defect at an interface between a gate insulating film and a semiconductor layer, and space charge is generated at the interface; thus the threshold voltage of the transistor is shifted.
In order to solve the problem of shifting the threshold voltage of the transistor, it is effective to reverse the polarity of potential which is applied to a gate electrode of the transistor when an EL element emits light, and to apply the potential and the revered potential to the gate electrode alternately. For example, in order to control the threshold voltage of a transistor, Reference 1 (Japanese Published Patent Application No. 2004-118132) proposes a DC current drive display device in which the threshold control voltage with reverse polarity for controlling the threshold voltage is applied to a transistor during a threshold voltage control period being provided separately from a period during which an EL element as a light-emitting element is emitting light.